In a display device such as a liquid crystal television, a surface light source device illuminates a liquid crystal panel which has image information, from the back face side of the liquid crystal panel. This makes the illuminating light pass through the liquid crystal panel to obtain the image information, and is emitted to the observer side, which makes it possible for observers to see images. On the other hand, the liquid crystal panel, because of its nature, has a limitation in light which can be effectively used. Therefore improvements for efficiently using light from the light source are demanded.
JP2010-217871A discloses an image source unit in which a surface light source, a prism sheet, an optical functional layer (a layer where light transmissive portions and light absorbing portions are alternately arranged), and a liquid crystal panel are layered in the order mentioned. This makes the directions of light entering the liquid crystal panel closer to the normal line of the panel face of the liquid crystal panel, whereby the use efficiency of the light is increased.
Japanese Unexamined Patent Application Publication No. 2011-501219 also discloses a structure in which a light source, a brightness enhancement film (a sheet where a plurality of prisms whose apexes face to the observer side are arranged), a reflection polarizing film, an LCF (a film where light transmissive portions and light absorbing portions are alternately arranged), a liquid crystal panel are arranged in the order mentioned. This makes it possible to make the direction of light emitted from the light source closer to the normal direction of the panel face of the liquid crystal panel, therefore it is said that the use efficiency of light can be increased. In addition, the light entered the LCF having a large angle to the panel face of the liquid crystal panel is absorbed by the light absorbing portions arranged in the LCF.